The present invention relates to respiratory, or ventilation, apparatus comprising a face mask and means for supplying pressurised air thereto, as well as to valves useful in such devices.
Non-invasive, mask-type ventilators, which include a face mask, pressurised air supply and valve, are known. These ventilators suffer from various disadvantages, primary amongst which is inflation of the abdomen via the oesophagus. As the stomach becomes inflated, this pushes up the diaphragm which, in turn, reduces lung volume and, concomitantly, tidal volume (Vt).
In addition, the ventilators of the prior art are not only cumbersome, but substantially restrict movement of the patient, as the pressurised air supply involves a length of tubing running from the mask to a fixed source of air or other suitable, breathable gas supply. The mask and tubing arrangement also tends to be heavy and somewhat inflexible, thereby putting further strain on the patient.
The substantial length of the tubing also tends to add somewhat substantially to the dead space. In this context, the dead space is that volume of air involved in the overall tidal flow which never comes into contact with gas exchange surfaces, in particular, the alveoli. When a patient is breathing normally, the dead space mainly comprises the trachea, nose and pharynx which, together, form about 150 ml of a total 600 ml tidal volume.
Using a mask of the prior art, the air supply tube may have a 10 mm internal radius and a length of 1800 mm, which provides an extra dead space, in addition to the 150 ml naturally occurring, of about 558 ml, thereby virtually doubling the tidal volume, as well as at least doubling the pressure required to effect satisfactory ventilation. It is such pressures which lead to problems with gas build up in the stomach.
One solution to the problem is to increase tidal flow and to create leaks in the mask to allow exhaust air, rich in carbon dioxide, to escape to help reduce the dead space problem. Another option is to provide a valve in the tubing to allow exhaled air to escape at an earlier stage. Both of these options still require substantial pressure to achieve satisfactory ventilation.
To achieve exhaust of CO2 in current masks, continuous positive flow and, therefore, pressure is required, even during exhalation of the patient. The pressure required to achieve this flow is around 8 cm H2O or greater. This forms the basic expiratory pressure which the patient faces at exhalation and which needs to be overcome in order for the patient to exhale. This pressure increases the work of breathing and distends lung volume, potentially beyond the need of the patient, while at least the same amplitude (the difference between peak inspiratory and trough expiratory pressures) is required to achieve adequate tidal volume, so that 0-10 cm H2O for a normal patient becomes 8-18 cm H2O (or more) for a patient using a face mask. The effects may be even more deleterious, as tidal volume of 0-10 cm H2O is greater than 8-18 cm H2O due to the lung pressure-volume curve.
One type of mask ventilator providing positive pressure ventilation, and which is non-invasive, is disclosed at page 609 of “Respiratory Care Equipment”, 2nd edition, 1999. A valve therein relies on natural exhalation, so that it is activated by expiration to cut off or reduce the supply of positive pressure, thereby enabling the patient to breathe out. In this type of ventilator, only one phase of the respiratory cycle, the inspiratory phase, is assisted and therefore active. This has the disadvantage that it is not possible to increase the respiratory rate above 4-30 cycles per minute, as there is no option to do anything other than rely on the natural expiration of the patient. As passive recoil generally requires a minimum of one second, this means that such ventilators cannot work at more than 30 cpm. There is an exhaust valve in the power unit, so that dead space is still a problem, and there is a single pressure chamber through which air from the blower passes, either to the patient during inhalation, or through an exhaust, during exhalation in order to reduce or cut off supply.
Swiss Patent no. CH685678 discloses an inhaler comprising a base-shaped container in which pressurised oxygen is stored. French Patent Application No. FR2446115 discloses a resuscitator, which fits over the mouth of the patient to supply air from a bulb, further comprising a tongue depressor. Pressure, created by a hand-operated airbag or bulb, forces air into the mouth of the patient.
U.S. Pat. No. 3,216,413 discloses a hand-operated concentric bellows-type resuscitator apparatus for artificial respiration without a hose, wherein one bellows is situated within a second bellows, and there is an arrangement of valves to enable assisted inhalation and exhalation of air from the patient's lungs at the appropriate pressures.
U.S. Pat. No. 3,939,830 discloses a manually operated resuscitator or dechoker for removing an obstruction from the throat of a patient. In and out strokes of a piston are used to inflate and deflate the lungs of the patient.
US Patent Application no. 2003/0111074 discloses a positive pressure hood comprising a power operated blower which forces air through a filter in order to generate a positive pressure within the hood. A one-way purge valve exists for the exhaust of exhaled gases. The apparatus is only suitable for maintaining a clean air supply, for instance in a laboratory or other contaminated environment, inside the hood and, therefore, is not suitable for respirating a patient
European Patent Application no. 0 352 938 discloses a powered respirator comprising a motor driven fan unit which draws air through an upstream filter unit, or alternatively, forces air through a downstream filter unit, for delivery to a face piece. The fan is triggered by a pressure sensor, which detects inhalation or exhalation by the patient leading to a corresponding assistance by the fan. Therefore, this device requires the patient to be breathing in the first place and cannot, therefore, be considered a respirator.